Refactor code/tests to apply almost rubocop rules

[nthachus]

• Change travis configuration to test on jRuby
• Update gemspec file to manage dependencies instead of Gemfile
• Use editor configuration file to maintain consistent coding styles
• Add development guide into README file
• Update git-ignore file to ignore vendor/bundle path
• Move class LdapFluff to the root of Gem lib-path
• Correct Rakefile to skip helpers while running unit-tests
• Update rubocop configuration file to use almost rules
• Move ldap_test_helper to the test root as a unit-test entrypoint
• Reformat all source-code
• Remove unnecessary active_support from dependencies
• Rename method is_in_groups? to user_in_groups? by rubocop recommendation
• Remove unnecessary instance variables by using config property
• Refactor all code/tests to apply almost rubocop rules
• Add optional parameter only into find_* methods to get one result
• Use base method *_filter (e.g. group_filter) to create Net::LDAP::Filter
• Optimize method users_for_gid to call find_group one-times only
• Add more tests to cover almost logic cases